Walter Atkinson said:
Some landmarks:
Peak CHT = approximately 40dF ROP. This is the point of maximum flame front speed, highest internal cylinder pressures and highest CHT.
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Thank you Walter for clearly outlining the the combustion ramifications of the mixture milestones such as Peak CHT, Peak EGT etc. I have printed them out and taped them on my wall as a simple guideline.

The understanding of the fuller combustion process in an internal combustion engine obviously cannot be derived from intuitive reasoning. It's a very complex science reliant on precise data aquisition.

Perhaps learning about the combustion process is a bit like learning to fly....you march up the mountain but you never reach the pinnacle.

But it is fascinating.

I had always thought that Peak EGT was a fuel efficient place to operate (without going LOP) and produced cooler CHTs than 50 df ROP but was harsh on the exhaust valve due to the high EGTs. Now you are saying that in fact exhaust valve temps are cooler at peak than at 50df ROP. Is that definitely correct.

Would you recommend Peak EGT as being best for normal cruise operations to promote engine longevity (for those without the means to operate LOP). If so what would be the max %age cruise power you would recommend at Peak EGT.
 
rv6ejguy said:
Diesels DON'T shed more heat than a gasoline engine!
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Ok, then explain why the diesels in aircraft have HUGE radiators and suffer a higher cooling drag penalty than aircooled aircraft engines.

Maybe we can get an answer to this one, since the last question about how a valve can get cooler while the EGT is getting hotter still remains unanswered.
 
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Captain Avgas said:
I had always thought that Peak EGT was a fuel efficient place to operate (without going LOP) and produced cooler CHTs than 50 df ROP but was harsh on the exhaust valve due to the high EGTs. Now you are saying that in fact exhaust valve temps are cooler at peak than at 50df ROP. Is that definitely correct.

Would you recommend Peak EGT as being best for normal cruise operations to promote engine longevity (for those without the means to operate LOP). If so what would be the max %age cruise power you would recommend at Peak EGT.
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Bob:

You are correct. Peak EGT is the least inefficient mixture IF one cannot operate LOP. <g>

40-50dF ROP is where the CHT is highest.

25dF ROP is the mixture that runs the exhaust valve the hottest. (Peak EGT is cooler on the exhaust valve than 25dF ROP.)

By the time the power is reduced to about 60-65%, you can't hurt the engine with any mixture you choose. Put it anywhere you want. Peak EGT is OK at that power and below (I use 60% to be conservative). Ever wonder why the OEMs suggest 65% power in cruise?? Kinda makes sense that the lowest common denominator in the pilot community can't screw it up! <vbg>
I will not be between 75 ROP and Peak above 65%.
Above 70%, I prefer to be richer than 125 ROP or leaner than 20dF LOP.
Above 75%, richer than 150-175dF ROP or leaner than about 35dF LOP.
Above 80%, richer than 200dF ROP or leaner than about 60dF LOP.

Those ranges keep the ICPs and CHTs under control and promote longevity.

I normally cruise my TNIO-550 Bonanza at 87-90% power at 80dF LOP or more. I like to go fast. I can do that with the hottest CHT below 370dF. (380 is my upper limit on CHTs.)

If I set the mixture as per the OEM recommendations at 50dF ROP, I can only get about 75% power before CHTs get above 410dF. That means I am substantially faster LOP with a cooler and cleaner engine, and MUCH cooler exhaust valves--the weakest link in our engines. There are more and more pilots (several thousand that I know of) now running their TC'd and NA engines this way with excellent results.

Check out <http://www.advancedpilot.com> if you would like to learn more about the details of engine management. Download "Deakin's Articles" from the TECH page for some reading on the subject.
 
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Walter Atkinson said:
Ok, then explain why the diesels in aircraft have HUGE radiators and suffer a higher cooling drag penalty than aircooled aircraft engines.

Maybe we can get an answer to this one, since the last question about how a valve can get cooler while the EGT is getting hotter still remains unanswered.
Click to expand...

Show me a properly executed diesel aircraft installation with HUGE rads. I'll grant you that some like the Diamond TwinStar does not use an efficient duct design, but the rads are hardly huge.

What are you comparing radiator sizes to? What do you base the higher drag assumption on? Do you have any historical references of similar liquid cooled installations being slower than an air cooled installation? Do you have an understanding of the pressure losses and heat rejection rates of air cooled engines vs. modern coolant radiators placed in a well designed divergent/ convergent duct? Do you have experience with liquid cooled engines in aircraft? This is not a matter of simple Delta T. It is a very complex subject with many variables.

In 2001 an Isuzu diesel powered 2 place, fixed gear homebuilt about the size of an RV12 demonstrated 118 knots TAS at 8000 feet on 68 hp. The rad and intercooler shared a common NACA duct with only 4 square inches of inlet area. Very efficient indeed for a turbocharged aircraft. Couldn't have been much a drag penalty here.

I'm sure you agree that since diesels have higher thermal efficiency, they must have less heat to dissipate for cooling compared to gasoline engines- unless you have found some way around the first law of thermodynamics.

For the valve temps, it would seem logical to conclude that the combustion process has an important influence on this factor.
 
percent power

Great info on this thread. I think I will be in good shape for LOP operations when I get flying since I have FI and a GRT EFIS/EIS for monitoring. I plan on working with Don at AFP to met my injectors matched properly.

Still confused about one thing after reading the Deakin articles and this thread:

A lot of this stuff seems to revolve around what percent of power you are at. My understanding is that the GRT EFIS will provide this reading based on interpolation from table values that I have to enter in, and that these values come from the Lycoming manual by matching up MAP and RPM. But this leaves out the mixture setting- isnt my power changing as I go from ROP to LOP? Surely the table values cant be correct for all mixture settings. So, how do I really know what percent power I am running at?

erich
 
Ron Roberts had some additional information for this thread.
Mathew Sharp

I believe the exhaust valves are some of the most unreliable parts of a combustion engine, giving engine designers and pilots headaches of many years (and probably for many years to come). If one is going to lean an engine, I can't think of anything more important than finding out what the affects of leaning are on an engine (and more specifically, on the exhaust valves). I think the answer to this question lies in the extensive research that has been done on this subject.

Report No. 754 "Operating Temperatures of a Sodium-Cooled Exhaust Valve as Measured by a Thermocouple" shows unequivocally that exhaust valves are affected by leaning. This report is worth reading and is free. See web
site:

http://naca.central.cranfield.ac.uk/reports/1943/naca-report-754.pdf

Also, this report shows the EGTs start to increase as the timing is advanced beyond 40 degrees TDC. This support the ideal that EGT's increase as damaging pre-ignition occurs. This is contrary to Walters's assertion that EGTs do not increase no matter how far the timing is advanced.

SAE Research Paper 920063, dated 1992, titled "Correlation of Exhaust Valve Temperature with Engine Reynolds Number in a 1.9L Engine," clearly shows the exhaust temperatures are hotter than the exhaust valve temperatures and indeed do significantly heat the exhaust valves during the exhaust stroke.
The exhaust valve temperatures were measured via thermocouples that were fitted through a small hole drilled down the stem of the exhaust valves.

SAE Research Paper 2006-01-0889, dated 2006, titled "Exhaust Valve Thermal Management and Robust Design Using Combustion and 3D Conjugate Heat Transfer Simulation with 6-Sigma Methodology," shows the exhaust valve temperature of a diesel engine is significantly heated by the exhaust gases during the exhaust stroke. This is in conflict with Walters's assertion that exhaust valves in diesel engines is lower than the exhaust gases.

SAE Research Paper 2000-01-0564, dated 2000, titled "Engine Valve Temperature Simulation System," states: "The temperature on (the exhaust
valve) top of head and under head are controlled by exhaust gas temperature.
They become the maximum near A/F = 13.5 where exhaust gas temperature becomes its maximum." This does not support Walters's assertion that exhaust valves reduce in temperature as EGTs go up near peak EGT.

SAE Research Paper 2003-01-0726, dated 2003, titled "Fatigue Analysis Methodology for Predicting Engine Valve Life," shows the exhaust gases 600'F higher than the exhaust valve temperature.

If you want to know conclusively what affects exhaust valve temperatures and their longevity, read some of these research papers and books published on this matter.
SAE Research Papers may be punched at:

http://www.sae.org/technical/papers/


I cannot find any research papers, published data or books that support the concept that exhaust valves are not heated by the exhaust gases. Engine health relies on a good and accurate understanding of how a pilot's actions affect the engine. Both EGTs and CHTs have a significant affect on the exhaust valve temperature and life!


Ron Roberts
Electronics International Inc.
63296 Powell Butte Hwy.
Bend OR 97701
Phone: 541-318-6060
Fax: 541-318-7575
 
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My own dyno testing and flight testing confirm that overly advanced timing does increase EGTs also as previously stated. This happens well above where maximum power is obtained however so would not impact most pilots with fixed magneto timing or a properly programmed FADEC.

Part of our flight testing program involves tests to find optimal ignition timing for various power settings and different AFRs to be able to compile maps for our EMSs. Instrumentation and equipment aboard allows us to vary spark timing and AFRs in flight and log EGT and AFR data via a wideband meter.
 
erich weaver said:
A lot of this stuff seems to revolve around what percent of power you are at. My understanding is that the GRT EFIS will provide this reading based on interpolation from table values that I have to enter in, and that these values come from the Lycoming manual by matching up MAP and RPM. But this leaves out the mixture setting- isnt my power changing as I go from ROP to LOP? Surely the table values cant be correct for all mixture settings. So, how do I really know what percent power I am running at?
Click to expand...


When ROP, power is a function of mass air flow. When LOP it is a function of FF only. Probably, although not certain, the Lycoming tables are correct (or at least close enough) for ROP operation but not applicable for LOP operation. On most of the NA Lycoming engines, 14.9 * FF = Hp when LOP. On most of the TC'd Lycomings, the mulitplier is 13.7 * FF =Hp.

Walter
 
**This does not support Walters's assertion that exhaust valves reduce in temperature as EGTs go up near peak EGT.**

It's not MY assertion. It is from the 1943 NACA study on the subject, confirmed by the 1966 Lycoming data, and rec-confirmed by data from the Carl Goulet Memorial Enigne Test Facility in about 2003. It's HARD data measurments, not a "contention."

Also, as far as the EGTs going up during pre-ignition, We have a number of data downloads of pre-ignition events from engine monitors, including EI instruments which show that in a running aircraft engine the EGT goes DOWN rather significantly during pre-ignition. These are OPERATIONAL issues for pilots, not an "I wonder what happens if you force the timing beyond the operational limits."

For EI to tell pilots in their manual to expect EGTs to go UP is contrary to what they will see on their EI monitor should they experience that event. That has serious implications.
 
Ron Roberts reply:

Walter,

Your hard data does not support your position. Let?s look at 1943 NACA Study, Report No. 754. Figure 9 shows the EGT increasing as the spark is advanced beyond 35 degrees. Figure 8 clearly shows the EGT hotter than the exhaust valve temperature and the exhaust valve temperature peaking at the same point that the EGT peaks (look at the hard data points depicted by the small round circles).

The 1966 Lycoming data you refer to is a Lycoming Service Training Manual for the IGSO-540 engine. Hardly a research document but I would like to review the data. Could you please provide this document for review?

Walter, please read the research papers I have referenced earlier. Pilots can benefit immensely by knowing the truth that both EGT's and CHT's affect exhaust valve temperatures and their longevity. My last interview with over 20 engine maintenance shops showed the top three maintenance problems were related to over heating of the exhaust valves. The results of this survey are documented in the "Pilot's Manual for Leaning and Diagnosing Engine Problems." If a pilot pays no attention to EGTs and believes the CHTs are the only parameter that affects the exhaust valves, he/she may be in for an expensive lesson.


Link to NACA Report:

http://naca.central.cranfield.ac.uk/reports/1943/naca-report-754.pdf

Link to "Pilot's Manual for Leaning and Diagnosing Engine Problems":

http://buy-ei.com/The_Pilots_Manual_by_EI.htm

Ron Roberts
Electronics International Inc.
63296 Powell Butte Hwy.
Bend OR 97701
Phone: 541-318-6060
Fax: 541-318-7575
 
Here's some data from an engine I have tested in my professional life. It's a liquid-cooled 4-cylinder 1.8L engine, but the fundamentals aren't too dissimilar for any engine. It's running 2000rpm, 60kPa MAP, one of the points this engine will advance past MBT without knock.

IgnitionvsEGT.gif


What you see is that for all intents and purposes, if you advance the ignition timing, your EGT's will drop. It models well as a quadratic, so I guess if you advance it far enough, you may see EGT begin to climb again, but it would be so advanced as to be into possible misfire territory.

Pre-ignition is essentially advanced ignition, but it isn't exactly that. For a start, the ignition source is different, the flame kernel will develop differently, the ignition delay will be different (less?) and possibly the burn rate may be higher. Add all of this together and it's hard to guess exactly what will happen, but it's more than likely true that the EGT's will be cooler than normal combustion. => low EGT's (and high CHT's) can suggest pre-ignition

Regarding exhaust valve temperature, it is true that it will be heated by the exhaust gasses passing by, but the surface area subject to significant flow velocity is quite low compared to the valve head face, which is subject to high gas temperature for much longer periods of time during the combustion cycle. Additionally, the valve spends much more of it's time in contact with the valve seat, which is one of the main mechanisms of valve cooling. It stands reasoning that it is more likely to be a function of cylinder head temperature than exhaust gas temperature.

Advancing your ignition will increase cylinder pressure and temperature, so your heat transfer to the cylinder head will be greater and generate higher CHT's (and valve temperatures)

Enrichening your mixture will do two things, depending on which side of your optimum mixture you are:

- If you are lean, it will add energy, hence also temperature and pressure so CHT's will rise, but so will EGT's because there is more energy in the system.

- If you are rich, then generally everything cools(EGT's and CHT's), since you are adding (thermal) mass, but not actually adding any energy because the energy available is largely dictacted by how much air is available for combustion.

A
 
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